This invention relates to a two-stage process for the removal of acid gases, boron compounds and particulates from a flue gas. The process is capable of meeting present and proposed emissions requirements.
Flue gases containing a mixture of acid gases, boron compounds and particulates are commonly found, for example, in the effluents from fiberglass melter furnaces. The acid gases in these effluents typically comprise hydrogen fluoride and sulfur oxides. The emitted boron compounds are typically boron oxides (B.sub.2 O.sub.3).
Prior art processes for treating these effluent gases have been unsuccessful for a variety of reasons. Liquid phase recovery systems using direct neutralization by calcium compounds or double alkali recovery using caustic scrubbing followed by calcium salt precipitation have resulted in the deposition of calcium fluoride in the scrubbers and eventual plugging because of the solubilization of calcium by borates.
Dry contact processes involving the sorption of acid gases on a solid reactive surface have also been unsuccessful, largely because of the need to quench the gas stream with water prior to the contact treatment. Such a quenching step is required to reduce the temperature and volume rate of the gas stream for both economy and corrosion considerations. Furthermore, the preferred mode of practicing the dry contact process for the sorption of acid gases taught in U.S. Pat. No. 3,808,774 requires a humidified gas stream. The problem with quenching gas streams containing boron oxides is that the normally solid, non-volatile oxides appear to hydrolyze to boric acid (H.sub.3 BO.sub.3) in the presence of moisture. Boric acid has been found to exert a measurable vapor pressure at typical effluent gas temperatures, and this gas-phase boric acid escapes from a dry recovery system and is exhausted to the atmosphere. Moreover, it has been found that this effect is greatly enhanced in an acid environment. It is believed that boric acid forms a complex with such substances as HF. The complex is significantly more volatile than boric acid alone and is relatively immune from sorption on typical dry sorption materials. Thus, it has been found that in such systems a white plume of boric acid and acid gases forms when the treated effluent gases are emitted to the atmosphere.